1. Field of the Invention
The present invention relates to an image processing apparatus for facilitating comparative reading of a plurality of medical images. The invention also relates to a computer readable recording medium on which is recorded a program for causing a computer to perform functions of the image processing apparatus.
2. Description of the Related Art
Heretofore, X-ray images have been obtained by irradiating radiation onto subjects for diagnosing diseases. Recently, in particular, a radiation image generation method that uses a radiation detector, such as stimulable phosphor, a flat panel detector (FPD), or the like, in place of a film, and directly derives a radiation image from the detector as a digital image has been used.
In order to find out a lesion from such images, it is important to compare past and present images of the same patient and observe a change between the images. For example, a plurality of image obtained for the same subject is displayed, arranged side by side, on a high-resolution display device and a diagnosis is performed by comparing them with each other. Where the past image and present image are obtained by radiographic machines of different models, or obtained by the same machine but under different imaging conditions, however, image characteristics (density value, contrast, tone, and the like) differ greatly between the images. There has been a case in which the difference in the image characteristics has become an obstacle to the comparative reading, and diagnostic accuracy and efficiency have been degraded.
Consequently, a technology for modifying image data using auxiliary information, such as imaging conditions and the like, attached to a radiation image so that the image characteristics correspond to each other between a plurality of images, thereby correcting the difference due to the difference in imaging conditions and the like, is proposed.
Such correction of difference is based on the assumption that the auxiliary information is attached to the obtained image data. The auxiliary information, however, is not attached to a radiation image obtained by an old-type radiographic machine, although it is attached to a digital image used in recent years. Otherwise, there may be a case that the auxiliary information is not sufficient due to the use of radiographic machines provided by different manufacturers. As such, the technology described above has problems that the modification for comparative reading can not be performed or even if a certain modification is performed, the result is not so effective.
Consequently, a method that includes the following steps is proposed as described, for example, in U.S. Pat. No. 6,744,849: selecting a reference image from a plurality of radiation images of the same region of the same subject; recognizing or specifying corresponding regions in the reference image and a similar image to obtain the image data; comparing the corresponding regions between the two images, and performing image processing so that the image characteristics of the reference image and similar image correspond to each other.
In the mean time, a mammography image is diagnosed focusing on a mammary gland region, and a chest image is diagnosed focusing on a lung field region. Therefore, use of such region as the specific region for matching the image characteristics, as in U.S. Pat. No. 6,744,849, allows the characteristics of the interest region to be matched without affected by a non-interest region that does not require any attention in comparison with the case in which the entire image is used.
The method for matching image characteristics between two images based only on a specific region, however, has the following problems.
Where a lesion is present in the specific region, the difference due to the lesion would also be eliminated. For example, in the case of a mammography image, a mass lesion is often found in the mammary gland region, so that if the image characteristics are matched based only on the mammary gland region, such lesion becomes less visible.
Further, the difference in thickness or amount of tissue due to difference in positioning would also be eliminated. For example, in the case of a mammography image, overlapping extent of mammary gland varies largely according to the positioning. More specifically, in certain positioning, the mammary gland does not stretch widely and the mammary gland mass appears more whitish on the image. With respect to two images that differ greatly in positioning, if a correction is made to match the image characteristics of the mammary gland region, the appearance of the mammary gland region corresponds to each other between the two images, but the appearances of the other regions differ greatly. That is, when the tissue structures are different in the first place, the difference should be left as it is. Despite this, difference arising from the difference in tissue structure is also eliminated.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide an image processing apparatus capable of improving diagnosis performance by matching the appearances of images while maintaining difference arising from the difference in tissue structure without being influenced by a non-interest region. It is a further object of the present invention to provide a computer readable recording medium on which is recorded a program for causing a computer to perform functions of the image processing apparatus.